Regeneron Pharmaceuticals Inc. (REGN:US) and Sanofi (SAN) and must assess how their experimental cholesterol drug affects brain function after U.S. regulators learned of adverse events associated with this new class of medicines.

The companies don’t know how the Food and Drug Administration learned of the side effect, Paris-based Sanofi said today in a regulatory filing. Sanofi, Regeneron and Amgen Inc. (AMGN:US) are among companies in late-stage testing of the therapies, which are designed to inhibit the PCSK9 protein associated with high levels of LDL, or “bad” cholesterol.